Because of their size and weight, large, wheeled construction vehicles such as front end loaders, road graders, backhoes and the like require power steering for proper vehicle control. The manual steering effort which would otherwise be required is excessive for even relatively short periods of time, notwithstanding a high level of individual operator physical strength. In addition, an occasional failure of one or more components critical to the normal power steering function will disable the vehicle unless means are provided for backup auxiliary power steering. In fact, it is nearly impossible even to controllably tow such a vehicle unless auxiliary power steering is available.
One solution to this problem is to equip the vehicle with a redundant power steering pump or to cause an auxiliary pump to be diverted to power steering use in the event of failure of the main power steering pump. A disadvantage of this approach is that it relies upon the availability of the vehicle engine and an engine failure would therefore make this approach useless. In addition, provision for such an auxiliary system must be made at the factory during the time of vehicle construction since it will likely require the provision of an additional pump mounting location. Examples of systems of this type are shown in U.S. Pat. Nos. 3,730,288 and 3,747,725.
Other ways in which auxiliary power steering may be provided are shown in U.S. Pat. Nos. 3,083,533 and 4,422,290. Both systems use a hydraulic accumulator (a vessel using gas under pressure to pressurize a contained quantity of hydraulic fluid) to provide auxiliary power steering fluid in the event of failure of the normal fluid source. The apparatus described in the first mentioned patent charges the accumulator using an electric pump driven from the vehicle battery while that described in the second patent charges the accumulator using an onboard implement pump. Since accumulators contain only a relatively small quantity of fluid, systems which use them become unreliable or non-functional after only a few steering movements. Such systems likewise are most satisfactorily installed at the factory. Even if they are charged by an electrically powered pump, their reliability depends upon the electric storage battery which, even if well charged, will become depleted over time. Examples of systems which use electrically powered auxiliary steering pumps are shown in U.S. Pat. Nos. 3,921,748; 3,847,243 and 3,820,620, among others. Of course, these electrically powered systems suffer from many of the same deficiencies described above. In fact, such a system will be useless if it is a battery failure which has disabled the vehicle.
Another way in which auxiliary power steering has been provided on such vehicles is by using the substantial kinetic energy of the moving vehicle as the motive power for the steering pump. Examples are described in U.S. Pat. Nos. 4,618,017 and 3,631,937. Of course, the most significant disadvantage of a system of this type is that it is not available when the vehicle is motionless. To aggravate the disadvantage, a motionless vehicle will likely require a higher steering effort than one which is moving.
While the systems described above have been generally satisfactory, they have failed to provide solutions to certain types of unusual steering requirements. For example, certain steering systems, e.g., those equipped with electric auxiliary pumps, accumulators or engine powered auxiliary pumps, may provide substantially the same ease of steering effort as the normal steering system. Therefore, the operator may forget that the normal steering system is disabled and the need for immediate repair may be overlooked.
In addition, these earlier systems have failed to recognize the manner in which an auxiliary power steering system may be constructed to meet recent requirements being imposed by some countries upon the functional characteristics of such vehicles. Specifically, some countries are requiring owners and manufacturers of such large vehicles to design them to be able to steer a measured, serpentine course even though specified vehicle components may be disabled and even though the vehicle may be under tow. Further, these earlier designs are not readily adapted to be provided in kit form for field installation.
An auxiliary power steering system which resolves the disadvantages described above would be a distinct advance in the art.